Many modern aircraft engines use substantial amounts of titanium (Ti) alloys in the fan and compressor sections. Since such sections of an aircraft engine are critical to the performance of the engine, the quality of the titanium alloys employed is a major concern. Jet engine failures have been directly attributed to the undetected presence of defects in the titanium components.
These defects include high density inclusions and hard alpha inclusions. High density inclusions (HDIs) are particles of significantly higher density than titanium and are introduced through contamination of raw materials used for ingot production. These defects are commonly molybdenum, tantalum, tungsten, and tungsten carbide. Hard alpha defects are titanium particles or regions with high concentrations of the interstitial alpha stabilizers, such as nitrogen, oxygen, or carbon. The worst defects are usually high in nitrogen and generally result from titanium burning in the presence of air during raw material production. Both types of defects discussed above are difficult to detect using conventional post-production inspection techniques.
The current industry accepted processes for producing premium high quality alloys, such as titanium alloys, are triple vacuum arc remelting (VAR) and a process involving an initial melting step in a cold hearth furnace followed by vacuum arc remelting (HM+VAR). The VAR process, even with the inclusion of premelt procedural requirements and post-production nondestructive test (NDT) inspections has proven unable to completely exclude hard alpha inclusions and has shown only a minimal capability for eliminating HDIs. The addition of cold hearth melting as an initial refining step in an alloy refining process has been extremely successful in eliminating the occurrence of HDI inclusions without the additional raw material inspection steps necessary in a triple VAR process. The HM process has also shown promise in eliminating hard alpha inclusions. However, the plasma cold hearth melting step is typically followed by a final VAR process. The VAR process gives known results but risks reintroducing inclusions or impurities into the ingot. To eliminate the final VAR step would allow the full realization of the benefits of the plasma cold hearth melting and would also provide significant economic advantages. A hearth melt only process could be carried out more economically than a triple VAR process or a process involving hearth melting followed by VAR. Additionally, a hearth melt only process gives improved elimination of melt-related defects.
The superiority of the HM process is derived from the fact that the molten metal must continuously travel through a horizonal water cooled hearth before passing into the ingot mold. Separation of the melting and casting zones produces a more controlled molten metal residence time which leads to better elimination of inclusions by mechanisms such as dissolution and density separation. Accordingly, there is a need in the art for a refining process involving a plasma cold hearth melting step only without the inclusion of a final VAR process. Problems associated with current industry processes, as well as aspects of the present invention are more fully explained in the paper by Clifford E. Shamblen (the Shamblen paper) entitled "Titanium Alloy Hearth Melt Only Technology Development" which appeared in the collection of papers entitled Titanium '95: Science and Technology. This paper is incorporated herein by reference.